New Rhizo­bia-di­atom sym­bi­osis solves long-stand­ing mar­ine mys­tery

Ni­tro­gen is an es­sen­tial com­pon­ent of all liv­ing or­gan­isms. It is also the key ele­ment con­trolling the growth of crops on land, as well as the mi­cro­scopic oceanic plants that pro­duce half the oxy­gen on our planet. At­mo­spheric ni­tro­gen gas is by far the largest pool of ni­tro­gen, but plants can­not trans­form it into a us­able form. In­stead, crop plants like soy­beans, peas and al­falfa (col­lect­ively known as legumes) have ac­quired Rhizo­bial bac­terial part­ners that “fix” at­mo­spheric ni­tro­gen into am­monium. This part­ner­ship makes legumes one of the most im­port­ant sources of pro­teins in food pro­duc­tion.

Sci­ent­ists from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men, Ger­many, now re­port that Rhizo­bia can also form sim­ilar part­ner­ships with tiny mar­ine plants called di­at­oms – a dis­cov­ery that solves a long-stand­ing mar­ine mys­tery and which has po­ten­tially far-reach­ing ag­ri­cul­tural ap­plic­a­tions.

In 2020, the sci­ent­ists trav­elled from Bre­men to the trop­ical North At­lantic to join an ex­ped­i­tion in­volving two Ger­man re­search ves­sels. They col­lec­ted hun­dreds of liters of sea­wa­ter from the re­gion, in which a large part of global mar­ine ni­tro­gen fix­a­tion takes place, hop­ing to both identify and quantify the im­port­ance of the mys­ter­i­ous ni­tro­gen fixer. It took them the next three years to fi­nally puzzle to­gether its gen­ome. “It was a long and painstak­ing piece of de­tect­ive work”, says Bernhard Tschitschko, first au­thor of the study and an ex­pert in bioin­form­at­ics, “but ul­ti­mately, the gen­ome solved many mys­ter­ies”. The first was the iden­tity of the or­gan­ism, “While we knew that the ni­tro­genase gene ori­gin­ated from a Vi­brio-re­lated bac­terium, un­ex­pec­tedly, the or­gan­ism it­self was closely re­lated to the Rhizo­bia that live in sym­bi­osis with legumes”, ex­plains Tschitschko. To­gether with its sur­pris­ingly small gen­ome, this raised the pos­sib­il­ity that the mar­ine Rhizo­bia might be a sym­biont.

Spurred on by these dis­cov­er­ies, the au­thors de­veloped a ge­netic probe which could be used to fluor­es­cently la­bel the Rhizo­bia. Once they ap­plied it to the ori­ginal sea­wa­ter samples col­lec­ted from the North At­lantic, their sus­pi­cions about it be­ing a sym­biont were quickly con­firmed. “We were find­ing sets of four Rhizo­bia, al­ways sit­ting in the same spot in­side the di­at­oms”, says Kuypers, “It was very ex­cit­ing as this is the first known sym­bi­osis between a di­atom and a non-cy­anobac­terial ni­tro­gen fixer”.

The sci­ent­ists named the newly dis­covered sym­biont Can­did­atus Tecti­glo­bus di­at­omi­c­ola. Hav­ing fi­nally worked out the iden­tity of the miss­ing ni­tro­gen fixer, they fo­cused their at­ten­tion on work­ing out how the bac­teria and di­atom live in part­ner­ship. Us­ing a tech­no­logy called nanoSIMS, they could show that the Rhizo­bia ex­changes fixed ni­tro­gen with the di­atom in re­turn for car­bon. And it puts a lot of ef­fort into it: “In or­der to sup­port the di­at­om’s growth, the bac­terium fixes 100-fold more ni­tro­gen than it needs for it­self”, Wiebke Mohr, one of the sci­ent­ists on the pa­per ex­plains.